Aluminizing process and composition



R. W. MARTINI ETAL ALUMINIZING PROCESS AND COMPOSITION Filed Aug. 9, 1963 Eiclwm] WA IariinL James R, Darnell INVENTOIQS BY QQQM YM afi 'ys United States Patent O 3,345,197 ALUMINIZING PRGCESS AND COMPOSITION Richard W. Martini and James R. Darnell, Muskegon, Mich., assignors to Howmet Corporation, a corporation of Delaware Filed Aug. 9, 1963, Ser. No. 301,048 6 Claims. (Cl. 117-26) This invention relates to the art of aluminizing metal surfaces by diffusion of aluminum at high temperature into the surface portions of the metal whereby the metal is rendered more resistant to corrosion or oxidation at high temperatures and/or in corrosive atmospheres, such as in the atmosphere existing in a combustion engine, turbine, and the like. By the introduction of aluminum by diffusion into the surface of such metals as nickel, cobalt, and chromium alloys and high alloy steels of nickel and cobalt, heat shock erosion, corrosion resistance, and many other physical and mechanical properties of the metal are improved.

To the present, in the aluminizing treatment, the alloy part is heated to a temperature above 1000 C. and preferably to a temperature of about 1100 C. in a powder mixture of metallic aluminum and aluminum oxide or other inactive metal oxide, with or without a small amount of halide salt, s uch as ammonium chloride, for

about four to ten hours. The powder aluminum is usually employed in a particle size of less than microns and the aluminum oxide is of a particle size greater than 50 microns and more often within the range of 50 to 100 microns.

The aluminum diffuses into the surface, usually to a depth of about microns, depending somewhat upon the time and temperature of the aluminizing treatment, and the amount of aluminum in the diffusion layer will average from 1 to percent by weight, and preferably from 2 to 8 percent by weight with the concentration of aluminum decreasing from the surface inwardly toward the center whereby the amount of aluminum in the outer portion is sometimes illustrated in the nickel alloy as NiAl, while the inner portions containing the lesser concentration of aluminum can be illustrated by the formula Ni Al.

It is an object of this invention to provide still further improvements in the process of aluminizing wherein the physical and mechanical properties of the nickel, cobalt, chromium base alloys or high alloy steels are further improved, and it is a related object to provide a new and improved aluminizing process and composition for use in same.

More specifically, it is an object of this invention to provide a new and improved aluminizing process and composition for improving the physical and mechanical properties of nickel and cobalt base alloys.

These and other advantages of this invention will hereinafter appear and, for purposes of illustration, but not of limitation, an embodiment of the invention is illustrated in the accompanying drawing in which:

The drawing is a diagrammatic cross-sectional view of the surface portion of a nickel-base alloy which has been aluminized in accordance with the practice of this invention.

It has been found, in accordance with the practice of this invention, that when the aluminum oxide component, in whole or in part, in the aluminizing powder, is of a particle size less than microns and preferably less than 10 microns, and more preferably more than 50 percent thereof less than 5 microns, the fine colloidal particles of aluminum oxide diffuse with the aluminum metal into the surface of the alloys to provide aluminum oxide inclusions substantially uniformly distributed ICC through the surface portions of the metal part with most of the inclusions being concentrated in the outer portion of the diffusion coating.

It is desirable to formulate the powder mixture with at least 50 percent by weight of the aluminum oxide component and preferably substantially percent by weight of the aluminum oxide component having a particle size of 20 microns or less, and more preferably with at least 50 percent of the aluminum oxide component having a particle size of 5 microns or less. During the normal diffusion process, the fine particles of aluminum oxide are carried into the surface of the metal part 10 to deposit as aluminum oxide inclusions 12 in the diffusion layer 14, thereby to provide a diffusion layer containing from 1 to 15 percent by weight of aluminum metal 16 with inclusions 12 of aluminum oxide powder in the diffusion layer. The amount of such inclusions will depend upon the aluminum oxide powder particle size, the time and temperature employed in the aluminizing process and the amount of aluminum oxide powder of fine particle size in the powder mixture.

The following will illustrate the practice of this invention:

The alloys parts are embedded in the powder and loaded into a retort in which they are maintained at 1100 C. for four to five hours. Upon removal from the retort, the parts are coated with an aluminized layer having a thickness of approximately 10 mils and containing an average of about 5 percent by weight aluminum metal and with aluminum oxide particles uniformly distributed as inclusions in the diffusion coating. The resulting part is characterized by good corrosion and oxidation resistance at temperatures up to 1000 C.

Example 11 Alloy composition:

Elements- Percent by weight Ni 70.0 Cr 12.0 W 5.0 A1 5.0 Mo 3.5 Ti, Nb, Ta 2.5 Fe, C, Mn, Si Balance Aluminizing powder:

Aluminum metal powder of less than 5 microns 5 Aluminum oxide of more than 50 microns 44 Aluminum oxide of less than 10 microns 50.7

The alloy parts are packed with the aluminizing powder in a retort and heated to a temperature of 1100 C. for about four hours. The resulting parts will have a diffusion layer of about 2 mils in thickness containing about 5 percent by Weight of aluminum, and the diffusion layer also contains inclusions of aluminum oxide substantially uniformly distributed throughout the outer portion thereof.

Example 111 Alloy composition:

Elements- Percent by weight Co 60.0 Cr 20.0 W 10.0 Nb 2.0 Ni 1.0 Fe, C, Mn, Si Balanc Aluminizing powder:

Aluminum metal powder of less than 5 microns 5 Aluminum oxide of less than 20 microns 95 The aluminizing process is substantially the same as that for Examples I and II.

The improvements secured by the practice of this invention will hereinafter be illustrated by reference to tests performed on alloys of the following compositions aluminized with powder mix A, which represents the practice of this invention, and a powder mix B formulated to contain aluminum oxide particles of a larger size but still smaller than the 50 to 10 micron size aluminum oxide particles heretofore employed in commercial practice.

Example IV Metal alloy (Udimet 500 nickel-base alloy):

Elements- Percent by weight C 0.08 Mn 0.75 Si e 0.75 Cr 19.0 C 19.5 Mo 4.0 Ti 2.9 A1 2.9 Fe 4.0 Ni Balance Example V Alloy composition (Inco 7130) Elements Percent by weight C 0. l2 Mn 0. 15 Si 0.4 Cr 13 .0 Mo 4.5 Ti 0.6 Al 6.0 Fe 1.0 Cb 2.25 Ni Balance Aluminizing powder mix:

Aluminum metal powder of less than 5 microns 5 Aluminum oxide 95 Having the following particle size distribution:

Percent by Weight Particle Size Composition Composition Less than 325 mesh (44 microns). 100 95-100 Less than 30 microns 100 75-94 Less than 25 microns 100 66-88 Less than 20 microns 100 56-77 Less than 15 microns 100 43-66 Less than microns 96-98 28-47 Less than 8 microns. 92-95 21-41 Less than 6 microns 79-83 13-33 Less than 5 microns. 68-73 10-28 Less than 4 microns. 55-59 8-23 Less than 3 microns. 40-42 3-17 Less than 2 microns 23-27 2-11 tions A and B for fivehours at a temperature of 1975 to 2000" F. The resulting coated parts were subjected to a standard oxidation-erosion test which. may be described as follows:

The test parts, having an air foil configuration and treated in accordance with Examples IV and V, were rapidly rotated and subjected to rapid heating to the test temperature of 1800 F. followed by fast cooling by means of an aspirated water spray to approximately 300 F. The specimens from each example were mounted together in order to uniformly expose all parts to exactly the same conditions of treatment for more accurate comparison of the resistance to the oxidation-erosion conditions. The following are the results that were obtained:

ALLOY OF EXAMPLE IV It will be seen from the results obtained with the alloy of Example V that composition B was etfective to increase the resistance of coated parts about two-fold over the uncoated metal parts, and that a further two-fold increase was achieved by treatment in accordance with the practice of this invention. With the alloy of Example IV,

a similar two-fold increase was observed when aluminized with composition B by way of comparison with the uncoated product, and the resistance to erosion was further decreased by 20 to 30 percent by the inclusion of aluminum oxide into the surface as obtained by the use of the powder composition A.

The foregoing is indicative of the improvement obtained by the inclusion of aluminum oxide into the treated surface in accordance with the practice of this invention. The reasons for the improvement obtained by the aluminum oxide inclusions have not yet been established. Nevertheless, such aluminum oxide inclusions have been determined to be present by microscopic analysis, and it has been established that such inclusions are important in It will be understood that changes may be made in the details of formulation and conditions of treatment without departing from the spirit of the invention, especially as defined in the following claims. 7

We claim: 1. In the method of aluminizing parts made of alloys and high alloy steels based upon at least one of the metals cobalt, nickel, and chromium, the steps of packing the parts in a stationary bed of a powder mixture of alumi-: num metal and aluminum oxide in which the aluminum oxide is of a particle size less than 20 microns with the aluminum metal being present in the amount within the range of 2 to 10 percent by Weight and heating the pack to elevated temperature whereby aluminum oxide particles are entrapped in the coating.

2. The method as claimed in claim 1 in which the metal part and aluminizing powder are heated to a temperature range of l0001100 C.

3. The method as claimed in claim 1 in which the metal part and aluminizing powder are heated to a temperature of about 1100 C. for four to five hours.

4. The method as claimed in claim 1 in which the metal part and the aluminizing powder are heated to a temperature above 1000 C. for a time Within the range of 1 to 15 hours.

5. The method as claimed in claim 1 in which the aluminum metal is present in an amount within the range of 4 to 6 percent by Weight of the aluminizing powder mixture.

6. The method as claimed in claim 1 in which at least 50 percent of the aluminum oxide component is of a particle size less than 5 microns.

References Cited UNITED STATES PATENTS 2,927,043 3/1960 Stetson 1061 3,096,205 7/1963 De Guisto 117 107.2 3,102,044 8/1963 Joseph 117 22 3,151,994 10/1964 Adlassnig 106-286 3,157,532 11/1964 Galmiche 117 107.2 3,178,308 4/1965 Oxley et a1 117-107 3,191,252 6/1965 Webbere 117:1'31

ALFRED L. LEAVITI, Primary Examiner. A. GOLIAN, Assistant Examiner. 

1. IN THE METHOD OF ALUMINIZING PARTS MADE OF ALLOYS AND HIGH ALLOY STEELS BASED UPON AT LEAST ONE OF THE METALS COBALT, NICKEL, AND CHROMIUM, THE STEPS OF PACKING THE PARTS IN A STATIONARY BED OF A POWDER MIXTURE OF ALUMINUM METAL AND ALUMINUM OXIDE IN WHICH THE ALUMINUM OXIDE IS OF A PARTICLE SIZE LESS THAN 20 MICRONS WITH THE ALUMINUM METAL BENG PRESENT IN THE AMOUNT WITHIN THE RANGE OF 2 TO 10 PERCENT BY WEIGHT AND HEATING THE PACK TO ELEVATED TEMPERATURE WHEREBY ALUMINUM OXIDE PARTICLES ARE ENTRAPPED IN THE COATING. 